This invention relates to fabrication techniques for producing micro-miniaturized electronic elements, more specifically to lift-off masking methods and other masking methods, still more specifically to a device fabrication method using sacrificial layers that are not adversely affected by radiation.
In the electronic industry reduced costs and higher operating performances of semiconductor devices are achieved by making the elements of the device smaller and closer together. This miniaturization inherently imposes a requirement for smaller and better defined interconnection metallurgy to connect the various active and passive elements of an integrated circuit device into an operative relationship.
Initially the interconnection metallurgy was fashioned by depositing a blanket layer of a metal on the surface of the substrate, depositing, exposing and developing a resist layer to define the desired metal pattern by leaving exposed the unwanted portions. The exposed areas were then removed by etching leaving the desired metallurgy pattern. As the metallurgy system of the integrated circuit devices became more miniaturized, new techniques known as lift-off, masking, stencil method, or expendable mask method was developed. U.S. Pat. Nos. 3,873,361; 4,004,044; 4,035,276; 4,090,006; and 4,202,914 disclose and claim various modifications of lift-off masking techniques. In lift-off technology a first sacrificial layer, usually of an organic polymeric material, is deposited on the substrate, possibly in combination with an overlying inorganic material, and the desired metallurgy pattern formed in the sacrificial layer using photolithographic techniques to define the openings. In order to provide good definition, necessary in micro-miniaturized metallurgy, it is usual to remove the exposed areas of the sacrificial layer with sputter etching techniques. A blanket layer of the desired metal is then deposited over the first layer after the resist layer has been removed. Thereafter the sacrificial layer is exposed to a solvent and the layer removed along with the overlying metal portions. The metal areas that are in direct contact with the substrate, which form the metallurgy, remain.
An acute problem with such lift-off fabrication techniques is that the underlying sacrificial mask layer can be difficult to dissolve and remove. Radiation causes cross-linking of the organic polymer which makes it more stable and harder to dissolve. The action of the solvent is unavoidably hampered by the overlying metal. Also the solvent selected must not adversely affect the associated layers and metallurgy of the device. With such constraints the cross-linking of the organic polymer material of the sacrificial layer significantly increases the etching time required for removal of the sacrificial layer. This makes the process expensive and also increases the probability of degrading the associated permanent device structure. Other related fabrication processes of electronic elements such as ion implantation in semiconductor devices, and metallurgy fabrication in the packaging technology are affected by the increased difficulty of etching or removing the masking layer after it has been exposed to radiation.
Radiation is involved in many processing operations associated with electronic device technology, i.e. sputter etching and deposition, vapor deposition using an electron beam for source vaporization, and ion implantation. In all of these operations a masking step is inherently involved.
In lift-off masking techniques it is known to use a polyaryl sulfone material as the material for the sacrificial mask. The material is commercially available and is derived from 2,2-bis(4-hydroxy-phenyl)propane (bisphenol A) and 4,4'-dichlorodiphenyl sulfone. The aromatic polysulfone polymer is stated to be relatively stable in radiation environments and can be dissolved with methyl pyrollidone (NMP). NMP is a desirable solvent because it effectively dissolves polysulfone polymer material, and its adverse effects on other parts of the electronic element is minimal. However, exposure of the polyaryl sulfone polymer to high energy radiation environments makes the material quite resistant to solvents, which is believed to be the result of cross-linking of the polysulfone polymer. This effect has been found to be so pronounced that the lift-off technique cannot use high energy radiation steps because the resultant polysulfone dissolution stage becomes inordinately long.